Such tunnels generally include means for conveying objects, heating or cooling means disposed along the side walls of the tunnel, and means for blowing a gaseous fluid into the tunnel at a predetermined temperature and along a direction which is essentially perpendicular to the direction along which the objects are conveyed.
The gaseous fluid, e.g. air, may be blown in to such a tunnel either for cooling or else for heating the objects being conveyed, and thus although the present invention is described essentially with reference to examples in which objects are heated, it should be understood that the invention is equally applicable to cooling.
Numerous applications may be concerned, and with respect to cooling, mention may be made of bottles of glass which need to be cooled in controlled manner after annealing, mainly for the purpose of releasing stress, and with respect to heating, mention may be made of shrinking a heat-shrink sleeve or length of sheath onto an object, in which case the sleeve is threaded loosely over the object to be decorated and/or protected, and is then heated to a temperature higher than the softening temperature of the film from which it is made so as to cause it to shrink onto the said object.
The invention relates more particularly to heating and shrinking on heat-shrink sleeves, however it must be understood that this is merely a particular application of the method of controlling temperature in accordance with the present invention and that the invention is not limited to this particular application.
It is now common practice to use certain types of plastic film optionally to print on them, and then form them into tubes by bringing together two edges of a strip and bonding them to each other, said tube then serving to decorate and/or protect an object, or more particularly the packaging a product.
Thus, receptacles such as bottles, aerosols, flasks, food cans, and other packaging objects are being provided more and more frequently with a protective sleeve or sheath, or a tamper-proofing ring, made of heat-shrink plastic material. The sleeve is placed around the receptacle, and after external heating to a temperature higher than its softening temperature, it should fit closely around the outline of the receptacle with a minimum of deformation. Such heat-shrink sleeves are made from plastic films (generally polyvinyl chloride films) to which memory is imparted during manufacture, and such films are generally referred to as heat-shrink films. These films are generally stretched essentially in the circumferential direction of the objects to be coated so as to acquire a memory (or shrink percentage) of up to 70% in the stretch direction (with the films commonly in use having a memory lying in the range 50% to 60%). Memory in the longitudinal direction, i.e. corresponding to the height of the length of sheath is only about 3% to 7%.
In order to impart memory to a film of plastic material (of polyvinyl chloride, polystyrene, or polyester, for example), the film must be heated to a very accurate temperature which is generally selected to be less than the vitreous transition point of the plastic material separating its amorphous zone and its elastic zone, while simultaneously subjecting the material to transverse and/or longitudinal traction. Heating softens the film, thereby allowing its molecules to creep and thus enabling the initial dimensions of the film to be increased, with a corresponding reduction in the initial thickness of the film.
Such films which are generally printed on and/or decorated to serve as labels on coated objects are said to be "mono-oriented" or "mainly mono-oriented". If transparent polyvinyl chloride films are used, printing may be performed on the inside surface, thereby providing a shiny external appearance and simultaneously protecting the printing against any risk of being rubbed off. In addition to the decorative aspect, this may be useful not only for tamper-proofing the contents, but also for setting up a barrier, e.g. for reducing loss of perfume from polyproylene packaging, or a loss of carbon dioxide gas from carbonated drinks in packaging made of polypropylene terephthalate.
Such an application has become widespread in the sale of consumer goods since it offers a wide range of decorating options, including the possibility of reproducing photographs, and it can be used for objects having a very wide variety of shapes.
Although the techniques of manufacturing heat-shrink films, of printing on them, of forming them into tubes, and of placing sleeves around objects or packages have already been substantially mastered, the same is not true of the operation of shrinking said sleeves, and shrinking becomes particularly difficult when the object or package has an irregular section which is triangular, square, or rectangular in shape, including faces having convex and/or concave zones.
It is essential for shrinking to take place uniformly around the object or package, i.e. without folding, wrinkling, or puncturing the sheath, and without deforming the printing provided on the film, particularly since the printing need not only be decorative, but may also have a direct effect on utilization (reading bar codes, printing legally-required messages, or instructions for use, for example).
The difficulties encountered in mastering the shrinking operation are largely due to problems which are essentially thermal, since the temperature to which the film is heated must be simultaneously accurate, constant, and uniform over the entire area of the film, while also being as low as possible.
Firstly, the temperature must be accurate.
There are only a few degrees between the elastic zone and the amorphous zone, and each plastic film has its own particular softening point and separation temperature between its elastic zone and its amorphous zone depending on the nature of the film and its formulation. In order to shrink the film, it is essential to know the temperature which corresponds to the beginning of the amorphous zone since the film restores its memory so long as its temperature remains below the temperature marking the beginning of the amorphous zone, with at least some of its memory acquired in the elastic zone being lost should this temperature be exceeded.
For example, with a film for which the amorphous zone begins at 110.degree. C. and having a memory which gives it a capacity to shrink by 50%, the full 50% shrinkage cannot be obtained if the temperature of 110.degree. C. is exceeded. Under such circumstances, it would no longer be possible to shape such a film in satisfactory manner on an object whose cross-sectional shape requires 50% shrinkage in the film. Numerous types of object would thus be barred from being coated by means of a heat-shrink sleeve.
The temperature must be constant.
The person skilled in the art is aware of shrinkage curves for films as a function of temperature (see FIG. 5, and corresponding longitudinal shrinkage curve RL and transverse shrinkage curve RT). From these curves, it can be deduced that if the film is subjected to a temperature of 80.degree. C. at some given instant (point A), then a shrinkage percentage of about 32% can be obtained in the transverse direction, but if at some other instant the temperature of the film is no more than 75.degree. C. (point B), then the portion of the film which has been subjected to this lower temperature will shrink by only 20% in the transverse direction. Such a large difference in shrinkage percentage (or in shrinkage rate) for so small a temperature difference clearly demonstrates that it is essential for the temperature to be constant.
The temperature must be uniform over the entire area of the film.
Referring to the same shrinkage curves, and assuming that the object has a simple right cross-section, imagine that two different points (A and B, FIG. 5) on the film are respectively at 80.degree. C. and at 75.degree. C., then the shrinkage will not be uniform between those two points on the film, thereby distorting or deforming the printing on the film.
It should also be observed that in spite of having a temperature which is both constant and uniform, there can also be heat loss from the film making contact at some points with the object to be coated while not making contact at other points, for example when the object has a concave face. By making contact with the object, heat is transferred from the film to the object, thereby reducing film temperature and giving rise to the consequences mentioned above.
Finally, the temperature should be as low as possible.
For a film having a high degree of mono-orientation, it may be possible to obtain 50% transverse shrinkage for only 7% to 8% longitudinal shrinkage. However, if the temperature to which the film is heated significantly exceeds the temperature which allows 50% transverse shrinkage to occur, not only is it no longer possible to obtain the 50% in full, but also the shrinkage percentage in the longitudinal direction is increased. The ratio of longitudinal shrinkage to transverse shrinkage therefore increases and can reach a value which gives rise to uncontrolled deformation of the printing to such an extent that the position of the printing on the object becomes entirely random since the printing moves arbitrarily both horizontally and vertically. Under such conditions, it is not possible to magnify the image adequately in both directions prior to printing to compensate for shrinkage of the film.
The above considerations show that it is essential to be able to control the temperature in the tunnel used for heating purposes with a very high degree of accuracy.
Several means have been recommended in order to attempt to solve some of the difficulties outlined above. For example: the use of tunnels having multiple preheat-shrink zones with flexible air-blowing tubes (European patent number 0 058 602); or the use of inwardly directed folds around the perimeter of the sleeve to be shrunk and in contact with the surface of the object to be coated. Reference may also be made to French patent number 75 30 896.
More recently, the Applicant has proposed blowing a gaseous fluid between the object and the loose sleeve to be shrunk, thereby inflating the sleeve and holding it out of contact with the object to be coated, with the temperature of the gaseous fluid being chosen to be below the softening temperature of the film constituting the sleeve, thereby making it possible to bring the temperatures of the inside and outside faces of the sleeve progressively into equilibrium and to control the thermal gradient within the film so as to obtain contact between the film and the object at a desired instant (see French patent number 85 15 717, on this topic).
This solution is advantageous, but it does not solve the problem of controlling temperature in the tunnel in the zone occupied by the means for blowing in the gaseous fluid.
However, control of this temperature is crucial in order to shrink the sleeve effectively while raising its inside and outside faces simultaneously to a common predetermined temperature, and in particular the temperature marking the separation between the elastic zone and the amorphous zone of the film.
It is very difficult to obtain such control in the tunnels used by virtue of numerous external disturbances which continually alter the ambient temperature in the zone occupied by the blower means.
It is easy to blow in a gaseous fluid at a constant predetermined temperature, but it is practically impossible to stabilize the temperature in a given zone of the tunnel, in particular at the temperature selected for shrinking the sleeve.
The person skilled in the art must always counter external disturbances of diverse origins. The inside of the tunnel is subjected along its length to convection currents which keep moving its temperature zones (currents due in particular to the temperature and the number of objects passing along the tunnel and/or to air being sucked into the tunnel). The thermal inertia of the shrinkage system and of the object conveyor system should also be mentioned.
Finally, persons skilled in this art are aware of a real need for a technique enabling temperature to be controlled satisfactorily.
An object of the invention is to provide a method, and an apparatus for implementing the method, for accurately controlling the temperature in a tunnel which is open at both ends, for the purpose of solving the above-mentioned difficulties.
Another object of the invention is to provide a method and an apparatus which are both simple and reliable, and which in particular make it possible to obtain uniform shrinking of a heat-shrink sleeve without deforming the printing thereon and without forming folds, wrinkles, or punctures therein, regardless of the shape or size of the object to be decorated and/or protected.
A subsidiary object of the invention in the context of this particular application is to enable said controlled shrinking to take place at relatively low temperatures on the surface of the film, e.g. 100.degree. C., thereby avoiding many of the above-mentioned drawbacks of random shrinkage when performed at conventional temperatures lying in the range of 180.degree. C. to 250.degree. C., and also reducing energy consumption and the length of tunnel required for performing shrinking.